A big earthquake struck south central Pakistan earlier today. The quake's magnitude was 7.7, and it happened in an area so remote that it will likely be many days before we know its true toll. At the moment, the official death toll is in the dozens, but it's almost certain to climb. And joining us to talk about this earthquake is NPR science correspondent Richard Harris. Hi, Richard.

RICHARD HARRIS, BYLINE: Hi, Robert.

SIEGEL: How unusual is an earthquake in Pakistan, an earthquake this big?

HARRIS: Well, quakes are frequent in this area. This is in the intersection of several continental plates. And as they slide past one another, or as one plate dives under another one, they do generate earthquakes. But that said, this earthquake is surprisingly large for the region. I talked to Roger Bilham at the University of Colorado, and he told me that one chunk of Pakistan moved something like 10 feet relative to the other part of Pakistan during this quake. And his very rough calculation suggests that a quake of this magnitude should only happen every, like, 800 years or so. And there, of course, there have been a few big ones in this part of South Asia in recent years, but not on this fault zone.

SIEGEL: And, Richard, remind us when we say a 7.7 magnitude quake. That is - compared to, say, a 6.7, that's 10 times greater.

HARRIS: That's right. Yes.

SIEGEL: And a hundred times greater than a 5.7 magnitude.

HARRIS: That's right, yes.

SIEGEL: It's pretty big.

HARRIS: Yeah.

SIEGEL: And so far, we won't know for a while exactly what damage has been done, but what kind of estimates are there of what's happened?

HARRIS: Well, the U.S. Geological Survey has a system to estimate the hazard of a quake based on ground motion, the number of people who are in the area and the vulnerability of the buildings. And this one triggered a red alert for the USGS. They said high casualties are probable, and the disaster is likely to be widespread. So even though the quake struck in a very remote part of Pakistan, the USGS figures some 200,000 people would have felt very strong to severe shaking. And many structures here are made of mud or stone, and as we've seen from previous earthquakes, those are very vulnerable and people often die in structures like that.

SIEGEL: Now there are also very intriguing reports that this quake created an island off the coast of Pakistan. How is that?

HARRIS: Well, I can't vouch for the photo that's making the rounds on the Internet, but it turns out this is possible. It turns out that buried in the sediment offshore from Pakistan, there's a type of ice that's made mostly of methane. And it turns out that during earthquakes, this methane can sometimes be released from its ice form and turn into a gas.

And when that happens, it can create a mud volcano, which can push its way though the surface of the ocean. These mud islands have actually formed in previous quakes in Pakistan, so it's possible another one has emerged during this quake. But the islands are low-lying and they don't last very long. They're - it's a big surprise though.

SIEGEL: Richard, in 2005, an earthquake of this size killed perhaps 100,000 people to the north of this quake, in Kashmir. Has Pakistan made any progress in upgrading its buildings to reduce the death toll from disasters like this one?

HARRIS: Well, Asif Khan from the University of Peshawar has told us that the government has made some progress. Seismologists are now mapping the earthquake risks to find out where all the faults are as best they can tell. And the government has adopted building codes to make sure structures are more able to stand up to a quake.

But, of course, building codes apply to new construction, and there are many, many old buildings that are still there and still pose a hazard to people. So some progress, but obviously still many vulnerable people.